1. Field of the Invention
The present invention relates to an optical system housing structure for use in an electrophotographic image forming apparatus such as a photocopying machine.
2. Prior Art
Electrophotographic image forming apparatus such as photocopying machines, laser beam printers, or the like are widely used in offices and other applications. As disclosed in Japanese Patent Publication No. 62-10423, most of such electrophotographic image forming apparatus have a housing structure which comprises a housing that contains an image forming mechanism including a photosensitive body and another housing that accommodates a scanning exposure optical system for optically scanning a subject and exposing the photosensitive body to an optical image of the subject. The housing which houses the scanning exposure optical system is placed on and fixed to the housing which houses the image forming mechanism. These housings are basically optically independent of, or isolated from, each other so that no light from a light source for applying scanning light to the subject in the exposure optical system will directly fall on the photosensitive body, thus preventing noise from being included in the final image. The independent housings can be installed and removed separately from each other.
The housing which houses the scanning exposure optical system, hereinafter referred to as the "optical system housing", including a glass platen for supporting the subject on its upper surface, tends to be heated to high temperature, making the user feel anxious about the condition of the scanning exposure optical system or sometimes making the user feel hot. Heretofore, external air is introduced into the optical system housing to cool the internal scanning exposure optical system and the glass platen so that they will not be unduly heated. The cooling air is forcibly delivered into the optical system housing by a fan. The optical system housing has an air inlet for introducing the cooling air and an air outlet for discharging the air after it has cooled the scanning exposure optical system and the glass platen. The air inlet is equipped with a filter for preventing dust from entering the optical system housing through the air inlet. The air outlet is usually left open without any filter.
It is known that the quality of images produced by the image forming apparatus is gradually degraded with time. The experience of the inventors indicates that the timedependent image quality deterioration takes place irrespectively of whether the image forming apparatus has been used frequently or not, and is caused due primarily to dust applied to optical surfaces of some optical components, such as mirrors, of the scanning exposure optical system. Small dust particles, which are invisible to human eyes, are suspended in air continuously for a long period of time, flow through narrow gaps, and are attached to the optical components. Even though those dust particles are small in size, they bring about a loss of light intensity when deposited to a certain thickness.
It is necessary that the optical components of the scanning exposure optical system be supported such that they can move for the scanning of the subject while remaining parallel to each other and to the photosensitive body or being kept at suitable distances from each other and from the photosensitive body. If the optical components are not properly held parallel to, or spaced from, each other and the photosensitive body, then the sharpness of generated images is gradually lowered, resulting in a image quality reduction. The optical system housing comprises surrounding side walls simply put together and joined together by arc welding. These surrounding side walls themselves do not have any means for keeping them perpendicularly to each other and also to a bottom panel. Therefore, certain jigs are necessary to position the side walls in order to hold them perpendicularly to each other and to the bottom panel when the side walls are joined together. The cost of the optical system housing thus constructed is relatively high because the side walls and the bottom panel are assembled using many jigs and the assembling procedure is complex. The complex assembling procedure is liable to impair the perpendicularity to be achieved between the walls. When the assembled walls are joined by arc welding, the desired perpendicularity may be lost due to the thermal strain of the walls. After the optical system housing has been assembled and welded, it has to be adjusted or finished for desired perpendicularity. The finishing process is tedious and time-consuming, and adds to an increase in the cost.
The various components are housed in the housings in accurate positions and with accurate attitudes. Unless the housings are coupled to each other in an accurate position and with an accurate attitude, however, the optical images cannot be formed on the photosensitive body by the scanning exposure optical system.
It is highly difficult to accurately align the upper edges of side walls of the housing which houses the image forming mechanism, hereinafter referred to as the "image forming mechanism housing", for supporting the optical system housing.
Heretofore, the optical system housing is fixedly mounted on the image forming mechanism housing after the upper edges of the side walls of the image forming mechanism housing have been adjusted in height.
However, a complicated position adjusting process is required to adjust the upper edges of the side walls of the image forming mechanism housing, and hence the housings cannot easily be assembled together. No positional reference is provided by either the image forming mechanism housing or the optical system housing when the upper edges of the side walls of the image forming mechanism housing are adjusted in height. After the adjusting process, the optical system housing is positionally adjusted back and forth and laterally on the image forming mechanism housing, and then the housings are joined to each other while being maintained in proper relative positions. Accordingly, it requires skill and time to assemble and join the housings, and the overall assembling and joining process is relatively costly.